One-electron reduction of some indole derivatives: Effect of 5-hydroxy substitution

Abstract

The reactions of e_aq⁻ with 3-substituted indole derivatives and their 5-hydroxy analogues viz., tryptamine, and 5-hydroxy tryptamine (serotonin); tryptophan and 5-hydroxy tryptophan; tryptophol and 5-hydroxy tryptophol; and also tryptophanol were studied using the pulse radiolysis technique. The transient spectra in all these cases showed maximum absorption at 345–350 nm, attributed to the indolyl radicals. The indolyl radical spectra of tryptamine, tryptophan, tryptophol and tryptophanol did not change significantly with pH, whereas the absorption maximum of the radical spectra of 5-hydroxy tryptamine, 5-hydroxy tryptophan and 5-hydroxy tryptophol changed into a broad red-shifted absorption at 375–380 nm in the alkaline region. By following these absorption changes as a function of pH, the radical pK_a of 9.3, 9.6 and 9.0 respectively, were estimated for 5-hydroxy tryptamine, 5-hydroxy tryptophan and 5-hydroxy tryptophol. The indolyl radicals at pH 7 decayed by second order radical–radical reactions with 2k values of 0.6–2.0 × 10⁹ dm³ mol⁻¹ s⁻¹ and the radicals were found to be reducing in nature. They reduce methyl viologen (MV²⁺) with rate constants of 1.5 to 6.4 × 10⁹ dm³ mol⁻¹ s⁻¹. The indolyl radicals react with oxygen with rate constants of 0.6–3.3 × 10⁹ dm³ mol⁻¹ s⁻¹. While the radicals from tryptamine, tryptophan, tryptophol and tryptophanol on reaction with oxygen did not produce any new absorption, the 5-hydroxy substituted compounds produced a new absorption in the 300 to 500 nm region with an absorption maximum at 400–410 nm, similar to indoloxyl radicals. The rate of formation of these newly absorbing radicals matched well with the decay of the indolyl radicals in the presence of oxygen and was independent of the pH and parent indole concentration. The oxygen radical adducts/indoloxyl radicals of all these indolyl radicals oxidise TMPD with rate constants of 1.5–8.7 × 10⁷ dm³ mol⁻¹ s⁻¹.